Carbon fibers are being increasingly used as fibrous reinforcement in a variety of matrices to form strong lightweight composite articles. Such carbon fibers are formed in accordance with known techniques by the thermal processing of previously formed precursor fibers which commonly are acrylic polymer fibers or pitch fibers. Heretofore, the formation of the fibrous precursor has added significantly to the cost of the carbon fiber production and often represents one of the greatest costs associated with the manufacture of carbon fibers.
All known commercial production of acrylic precursor fibers today is based on either dry- or wet-spinning technology. In each instance the acrylic polymer commonly is dissolved in an organic or inorganic solvent at a relatively low concentration which typically is 5 to 20 percent by weight and the fiber is formed when the polymer solution is extruded through spinnerette holes into a hot gaseous environment (dry spinning) or into a coagulating liquid (wet spinning). Acrylic precursor fibers of good quality for carbon fiber production can be formed by such solution spinning; however, the costs associated with the construction and operation of this fiber-forming route are expensive. See, for instance, U.S. Pat. No. 4,069,297 wherein acrylic fibers are formed by wet spinning wherein the as-spun fibers are coagulated with shrinkage, washed while being stretched, dried, and stretched prior to being used as a precursor for carbon fiber production. A key factor is the requirement for relatively large amounts of solvents, such as aqueous sodium thiocyanate, ethylene carbonate, dimethylformamide, dimethylsulfoxide, aqueous zinc chloride, etc. The solvents often are expensive, and further require significant capital requirements for facilities to recover and handle the same. Precursor fiber production throughputs for a given production facility tend to be low in view of the relatively high solvent requirements. Finally, such solution spinning generally offers little or no control over the cross-sectional configurations of the resulting fibers. For instance, wet spinning involving inorganic solvents generally yields substantially circular fibers, and wet spinning involving organic solvents often yields irregular oval or relatively thick "kidney bean" shaped fibers. Dry spinning with organic solvents generally yields fibers having an irregularly shaped "dog-bone" configuration.
It is recognized that acrylic polymers possess pendant nitrile groups which are partially intermolecularly coupled. These groups greatly influence the properties of the resulting polymer. When such acrylic polymers are heated, the nitrile groups tend to crosslink or cyclize via an exothermic chemical reaction. Although the melting point of a dry (non-hydrated) acrylonitrile homopolymer is estimated to be 320.degree. C., the polymer will undergo significant cyclization and thermal degradation before a melt phase is ever achieved. It further is recognized that the melting point and the melting energy of an acrylic polymer can be decreased by decoupling nitrile-nitrile association through the hydration of pendant nitrile groups. Water can be used as the hydrating agent. Accordingly, with sufficient hydration and decoupling of nitrile groups, the melting point of the acrylic polymer can be lowered to the extent that the polymer can be melted without a significant degradation problem, thus providing a basis for its melt spinning to form fibers.
While not a commercial reality, a number of processes involving the hydration of nitrile groups have been proposed in the technical literature for the melt spinning of acrylic fibers. Such acrylic melt-spinning proposals generally have been directed to the formation of fibers for ordinary textile applications wherein less demanding criteria for acceptability usually are operable. The resulting fibers have tended to lack the uniform structure coupled with the correct denier per filament required for quality carbon fiber production. For instance, the required uniform molecular orientation commonly is absent, surface defects and significant numbers of broken filaments are present, and/or an unacceptably high level of large voids or other flaws are present within the fiber interior. Even though "substantially void free" terminology has been utilized in some of the technical literature of the prior art with respect to the resulting acrylic fibers, satisfactory carbon fibers could not be formed from the same.
Representative, prior disclosures which concern the melt or similar spinning of an acrylic polymer to form acrylic fibers primarily intended for the usual textile applications include: U.S. Pat. Nos. 2,585,444 (Coxe); 3,655,857 (Bohrer et al); 3,669,919 (Champ); 3,838,562 (Park); 3,873,508 (Turner); 3,896,204 (Goodman et al); 3,984,601 (Blickenstaff); 4,094,948 (Blickenstaff); 4,108,818 (Odawara et al); 4,163,770 (Porosoff); 4,205,039 (Streetman et al); 4,418,176 (Streetman et al); 4,219,523 (Porosoff); 4,238,442 (Cline et al); 4,283,365 (Young et al); 4,301,104 (Streetman et al); 4,303,607 (DeMaria et al); 4,461,739 (Young et al); and 4,524,105 (Streetman et al). Representative prior spinnerette disclosures for the formation of acrylic fibers from the melt include: U.S. Pat. Nos. 4,220,616 (Pfeiffer et al); 4,220,617 (Pfeiffer et al); 4,254,076 (Pfeiffer et al); 4,261,945 (Pfeiffer et al); 4,276,011 (Siegman et al); 4,278,415 (Pfeiffer); 4,316,714 (Pfeiffer et al); 4,317,790 (Siegman et al); 4,318,680 (Pfeiffer et al); 4,346,053 (Pfeiffer et al); and 4,394,339 (Pfeiffer et al).
Heretofore, acrylic fiber melt-spinning technology has not been sufficiently advanced to form acrylic fibers which are well suited for use as precursors for carbon fibers. However, suggestions for the use of melt spinning to form acrylic fibers intended for use as carbon fiber precursors can be found in the technical literature. See, for instance, the above-identified U.S. Pat. No. 3,655,857 (Bohrer et al); "Fiber Forming From a Hydrated Melt--Is It a Turn for the Better in PAN Fibre Forming Technology?", Edward Maslowski, Chemical Fibers, pages 36 to 56 (March, 1986); Part II--Evaluation of the Properties of Carbon Fibers Produced From Melt-Spun Polyacrylonitrile-Based Fibers, Master's Thesis, Dale A. Grove, Georgia Institute of Technology, pages 97 to 167 (1986); High Tech--the Way into the Nineties, "A Unique Approach to Carbon Fiber Precursor Development," Gene P. Daumit and Yoon S. Ko, pages 201 to 213, Elsevier Science Publishers, B.V., Amsterdam (1986); Japanese Laid-Open Patent Application No. 62-062909 (1987); and "Final Report on High-Performance Fibers II, An International Evaluation to Group Member Companies," Donald C. Slivka, Thomas R. Steadman and Vivian Bachman, pages 182 to 184, Battelle Columbus Division (1987); and "Exploratory Experiments in the Conversion of Plasticized Melt Spun PAN-Based Precursors to Carbon Fibers", Dale Grove, P. Desai, and A. S. Abhiraman, Carbon. Vol 26, No. 3, pages 403 to 411 (1988). The Daumit and Ko article identified above was written by two of the present joint-inventors and contains a non-enabling disclosure with respect to the presently claimed invention.
In our copending U.S. patent Ser. No. 236,177, filed concurrently herewith, entitled "Improvements in the Formation of Melt-Spun Acrylic Fibers Which Are Particularly Suited for Thermal Conversion to High Strength Carbon Fibers" is disclosed a companion invention wherein the as-spun acrylic fiber product may be used to form carbon fibers of high strength. The products of the present invention tend to possess greater internal perfection than those of such companion application.
It is an object of the present invention to provide an improved process for the melt spinning of acrylic fibers which are particularly suited for carbon fiber production in the substantial absence of filament breakage.
It is an object of the present invention to provide an improved process for the melt spinning of acrylic fibers which possess an internal structure which is particularly well suited for subsequent thermal conversion to form quality carbon fibers.
It is an object of the present invention to provide an improved process for the melt spinning of acrylic fibers which possess an internal structure which is particularly well suited for subsequent thermal conversion to form quality carbon fibers having a relatively low denier per filament.
It is an object of the present invention to provide an improved process for the melt spinning of acrylic fibers which possess an internal structure which is particularly well suited for subsequent thermal conversion to form quality carbon fibers of a predetermined substantially uniform cross-sectional configuration which may be widely varied.
It is an object of the present invention to provide an improved process for melt spinning of acrylic fibers which are particularly suited for carbon fiber production wherein such acrylic fiber precursor formation is capable of being expeditiously carried out on a relatively economical basis.
It is an object of the present invention to provide an improved process for the formation of acrylic fibers which are particularly well suited for carbon fiber production wherein such spinning is carried out using a lesser concentration of solvents than was used in the prior art.
It is an object of the present invention to provide an improved process for the formation of acrylic fibers which are particularly suited for carbon fiber production requiring lesser capital requirements to implement than the prior art and being capable of operation on an expanded scale through the use of readily manageable increments of equipment.
It is another object of the present invention to provide novel acrylic fibers which are substantially void free when examined in cross section at a magnification of 2,000.times. and thereby possess a highly uniform internal structure which is particularly well suited for thermal conversion to carbon fibers.
It is a further object of the present invention to provide novel carbon fibers of good quality which are substantially void free when examined in cross section at a magnification of 2,000.times. having a predetermined substantially uniform cross-sectional configuration formed by the thermal processing of the improved melt-spun acrylic fibers of the present invention.
These and other objects as well as the scope, nature, and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.